It is known to use a variety of acceleration measuring devices called accelerometers, particularly for guidance of aircraft, spacecraft, and guided weaponry. A common form of accelerometer is dynamic, employing closed loop feedback to determine accelerations in a sensitive axis. Devices of this type may typically sense accelerations on the order of ten micro g (the acceleration of gravity, 32 ft/sec.sup.2), and are very expensive. In some applications, accelerations on the order of one thousand micro g are involved. This requires a different type of accelerometer. One form of accelerometer known to the prior art is a mass supported on a cantilevered beam, so that acceleration of the mass in the sensitive axis will cause a bending moment in the beam, resulting in a sensible strain. The sensing of strain in the beam has been achieved in a variety of ways. For instance, U.S. Pat. No. 3,411,361 describes the use of bonded resistor bridges disposed on the surfaces of a cavity within the beam to sense the strain therein. However, strain gages of the bonded resistor type typically have sensitivities on the order of two percent of full range, which is inadequate in many applications.
A more sensitive type of cantilevered mass accelerometer employs the variation in propagation time of a surface acoustic wave (SAW) on a piezoelectric beam, such as quartz. The strain induced by bending alters the acoustic velocity of the wave in the beam, which can be measured in a variety of ways, such as alteration of the frequency of an oscillator in which the frequency determination is principally dependent upon the acoustic velocity of the wave. Such a device is disclosed, inter alia, in U.S. Pat. No. 3,863,497. It is known that devices of this type when suitably designed are capable of sensitivities which are at least two orders of magnitude better than the sensitivities of bonded resistor strain sensors. Thus, accelerometers employing SAW devices may have sensitivities on the order of 0.001% of full range.
In some applications, it may be necessary to sense extremely high accelerations. For instance, in a crash recorder on a commercial airline, it would be useful to sense the deceleration of the aircraft during the moments preceding a crash and during the crash itself. This could provide useful information in determining the cause of the crash. Also, the impact of a meteorite on a spacecraft may typically provide accelerations way in excess of those which the normal accelerometer used for guidance can sense. At times, system weight and space constraints may preclude the use of several accelerometers in each axis in order to provide suitable ranges. However, it is difficult to provide accelerometers with a variety of ranges.
One characteristic of SAW devices which is utilized to advantage as is known in the art, is the fact that its output signal varies in frequency, rendering it easily adapted for use with digital processing circuits. Therefore, it may be desirable particularly where there are weight and space constraints, to utilize accelerometers employing surface acoustic wave effects in applications where digital computation is necessary and real time processing delay constraints preclude conversion of various analog signals to digital form.
Therefore, there is a real need in many applications for the features of an accelerometer which are achievable essentially only by the digital-compatible, highly sensitive SAW type of strain sensor.